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Inflow roll (known as transverse flow) causes an uneven flow distribution leading to the rise of the rotor disc:

  • A
    At the rear of the helicopter, and makes it roll to the retreating side.
  • B
    On the advancing side, and makes the helicopter roll to the retreating side.
  • C
    At the front of the helicopter, and makes it roll to the advancing side.
  • D
    On the retreating side, and makes the helicopter roll to the advancing side.

Refer to figure.
As the helicopter moves forward, the disc is tilted and coned. As a result, the relative air flow meets the disc at different angles.

At the front of the disc. The angle is shallow. This has the effect of reducing the induced flow which increases the angle of attack and therefore lift in this area.

At the rear of the disc, the angle is steeper and so a greater proportion of horizontal flow flows through the disc, adding to the induced flow, decreasing the angle of attack and therefore, lift is reduced in this area.

Looking at a counter clockwise rotor system from above the blade at the right of the Pilot will start flapping upwards and have the highest rate of flapping up at the front of the helicopter thereafter continue flapping up to the left side of the Pilot on the retreating side.

Once the blade passes the left of the Pilot it starts flapping down and has the maximum rate of flapping down at the rear of the helicopter and continues flapping until the blade is to the right of the Pilot at the advancing side.

This gives the retreating side of the blades a higher position relative to the advancing side and causes the helicopter to roll towards the advancing side. To correct for inflow roll the Pilot should simply apply cyclic towards the left in this case.


NOTE:
It’s advised to think that inflow roll always causes the helicopter to roll towards the advancing side and not to think of the direction of rotation of the main rotor.

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